(1) Field of the Invention
The present invention relates to a power plant for an aircraft, in particular a rotorcraft, and to a method of piloting said aircraft.
(2) Description of Related Art
Most presently-built rotorcraft are fitted with one or two free-turbine engines. Power is then taken from a low-pressure turbine that is known as a “free turbine”, which turbine is mechanically independent from the assembly comprising the compressor and the high pressure stage, including in particular a high pressure turbine, of the turbine engine. The free turbine of a turbine engine generally rotates at a speed lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reducing gearbox is needed to connect it to the main rotor of the rotorcraft that has a speed of rotation lying substantially in the range 200 rpm to 400 rpm: this is the main power transmission gearbox (MGB).
The temperature limitations of a turbine engine and the torque limitations of an main power transmission gearbox serve to define a utilization envelope for the turbine engine covering two normal utilization ratings for a turbine engine arranged on a single-engined or twin-engined rotorcraft:                a takeoff rating corresponding to a level of torque for the main power transmission gearbox and a temperature rise for the turbine engine that are acceptable over a limited length of time without significant degradation, this takeoff rating being defined by a maximum takeoff power (PMD) and by a utilization duration for said maximum takeoff power PMD, which is generally of the order of 5 minutes; and        maximum continuous rating, which rating is defined by a maximum continuous power (MCP) corresponding to about 90% of the maximum takeoff power PMD, and by a utilization duration for said maximum continuous power that is generally unlimited.        
On a twin-engined rotorcraft, the performance envelope also covers contingency power ratings during which extra power is used, but only when one of the two turbine engines breaks down:                a first contingency rating, defined both by a first supercontingency power PSU that is often equal to about 112% to 120% of the maximum takeoff power (PMD) and by a utilization duration for said first supercontingency power PSU that is generally of the order of at most thirty consecutive seconds, which first supercontingency power is conventionally usable up to three times during a flight;        a second contingency rating, the second contingency rating being defined both by a maximum contingency power PMU equal to about 105% to 110% of the maximum takeoff power (PMD) and by a utilization duration for said maximum contingency power PMU of the order of at most two consecutive minutes; and        a third contingency rating, the third contingency rating being defined both by an intermediate contingency power PIU that is substantially equal to the maximum takeoff power (PMD) and by a utilization duration of said intermediate contingency power PIU that is unlimited for the remainder of the flight after the breakdown of the turbine engine.        
Thus, the engine manufacturer defines a utilization envelope for the turbine engine, said utilization envelope comprising a plurality of ratings, each rating associating a power developed by the turbine engine with a utilization duration for said power.
Furthermore, temperature and mechanical constraints and above all the phenomenon of turbine blade creep can lead to the turbine engine being degraded to a greater or lesser extent depending on the rating. In order to guarantee safety in flight and that performance will be obtained, it is therefore essential to determine the maximum amount of damage that is acceptable for a turbine engine.
Thereafter, the overall utilization potential of the turbine engine is evaluated. Concretely, this amounts to defining a maximum number of flying hours, known to the person skilled in the art as time between overhauls (TBO), that the turbine engine is capable of carrying out as measured from its most recent overhaul or from its first use, depending on which situation is applicable. Once that maximum number of flying hours has been reached, the turbine engine is removed and then overhauled.
Below and for convenience, the term “most recent overhaul of the turbine engine” is used to designate as appropriate, either the first use of the turbine engine or else the most recent occasion on which it was, in fact, overhauled.
Thus, the engine manufacturer defines a utilization envelope for the turbine engine that is associated with some maximum number of flying hours, said utilization envelope corresponding to a plurality of ratings, each rating associating a level of power developed by the turbine engine with a utilization duration for that power level. Furthermore, the manufacturer associates a maximum number of flying hours with the utilization envelope.
It should be recalled that a turbine engine is usually provided with control means, and that information relating to the ratings of an envelope is stored in the control means. Under such circumstances, when the pilot of an aircraft requires a given rating to be used, the control means control the turbine engine, and in particular its fuel metering unit, so that the turbine engine responds to the given order.
Furthermore, in order for a rotorcraft to obtain authorization to fly in a given country, it will be understood that the utilization envelope and the maximum number of flying hours for the turbine engine(s) of the rotorcraft need to be certified by the official services in the country under consideration for a specified utilization spectrum. Such authorization is thus obtained only after thorough certification testing, which is very expensive.
Since such thorough certification testing of a turbine engine is performed in order to justify a utilization envelope associated with some maximum number of flying hours, it is not possible to use the turbine engine with a performance envelope that is different from the performance envelope that was initially authorized, without performing thorough certification testing, which, once more is very expensive.
It can be understood that a given turbine engine may correspond to a type of mission. Nevertheless, the turbine engine runs the risk of not having an optimized staging of the ratings of its utilization envelope for a mission of some other type.
For example, a life-saving mission involving winching requires a turbine engine to operate in accordance with a utilization envelope that is different from a utilization envelope that has been optimized for a mere ferrying mission.
Under such circumstances, a utilization envelope allows one type of mission to be carried out but, a priori, does not allow some other type of mission to be carried out, or at least does not allow it to be carried out in optimized manner.
Furthermore, regulations, and for example the JAR-OPS3 European Operational Regulations, require manufacturers to ensure aircraft safety, in particular during stages of takeoff and landing.
Requirements vary depending on the takeoff area. Existing regulations define various types of takeoff area such as a heliport, a helispot, and a platform. For example, the JAR-OPS3 regulations state that a platform is a takeoff area situated at least three meters above the surrounding surface, the platform being referred to as an “elevated” heliport.
Once a takeoff area has had a type allocated thereto, the surroundings of the takeoff area are also defined, which surroundings may constitute an environment that is hostile and not obstacle-free, for example.
Finally, the regulations define in particular the type of landing that is possible after one of the engines of a twin-engined rotorcraft has broken down. For example, the JAR-OPS3 regulations specify three so-called “performance” classes 1, 2, and 3.
Under such circumstances, the manufacturer must set up takeoff and landing procedures that make it possible to ensure that the aircraft is safe in compliance with the criteria required by the regulations, which procedures may vary from one aircraft to another, and as a function of the class associated with a takeoff area.
The manufacturer of an aircraft thus draws up such takeoff and landing procedures as a function of the capabilities of the aircraft.
It can be understood that a power plant having an unchanging utilization envelope gives the manufacturer little room to maneuver when drawing up optimized landing and takeoff procedures.
However, by optimizing landing and takeoff procedures, it is possible to maximize the maximum weight that the rotorcraft can transport.
As a result, an aircraft having a power plant that operates in accordance with an unchanging utilization envelope puts a limit on:                the missions that can be performed by that aircraft; and        the procedures that can be selected for landing and takeoff, and consequently the maximum weight that can be transported.        
Conventionally, when developing an aircraft, a manufacturer selects a power plant having a utilization envelope suitable for satisfying the usual requirements of most of that manufacturer's clients, and then establishes the best takeoff and landing procedures made possible by using that power plant.
It is also possible to use a power plant that is overdimensioned in order to be capable of carrying out multiple missions. Nevertheless, such a power plant is both expensive and also penalizing from a weight point of view.
According to document FR 2 878 288, it is possible to modify a utilization envelope of a turbine engine by modifying the maximum number of flying hours.
According to document FR 2 888 288, starting from an initial utilization envelope, an alternative envelope is drawn up. The conversion from the initial utilization envelope to the alternative envelope is performed without modifying the number of maximum flying hours for the turbine engine, but by lowering the value of an initial envelope parameter. For example, the power of a given rating is increased, but the utilization duration of that rating is decreased.
The state of the art also includes documents EP 1 281 846 and FR 2 602 270 that mention the possibility of reevaluating the limits of an engine in an emergency.